Australia’s wildlife heritage

Wasps are generally considered dangerous insects. In fact they are among the most significant of all insects as far as mankind is concerned, owing to their predations on other insects. This is an important factor in checking the world’s already immense insect population, writes Nutan Shukla.

PARASITIC wasps, like the digger-wasps of Arizona, are dangerous to large spiders. In a seemingly one-sided struggle, the small but agile wasps are able to outmanoeuvre and overpower the large, hairy tarantula spider. Found in South Africa they are the world’s largest spiders, growing to 7-8 inches in length. They are highly predatory creatures and do not hesitate to prey even on small birds, hence they are also known as bird-eating spiders. On the other hand, digger-wasp does not grow more than two inches in length, but still overpowers spiders which are three to four times its size.

Whenever a female digger-wasp is about to lay eggs, it digs a burrow in the ground and then searches for a spider (each species of wasp attacks only specific kinds of spider). Solitary by nature, when it finds its prey it engages it in a short battle, seeking an opportunity to use its sting, which is located at the tip of its abdomen.

These wasps have an extremely slender waist between the thorax and the abdomen (hence the expression wasp-waisted), and this gives them the ability to make extremely acrobatic and precise movements. While holding the spider (and evading the fangs), the wasp tucks its abdomen forward beneath its legs and, using an instinctive understanding of the anatomy of spiders, inserts its sting into the spider’s body at the location of the main nerve ganglion — the invertebrate equivalent of the brain — and injects a dose of venom.

Unlike the venom of many animals, that of the wasp acts with great precision on the central nervous system of the spider. It paralyses the muscles used in movement, thus completely immobilising the spider, but it does not stop either the breathing or the blood circulation of the spider.

Paralysed but still very much alive, the spider is carried by the wasp back to its nest and carefully placed in the burrow. Now the female lays her egg on it. Only one egg is laid in one burrow because the stored food is sufficient for only one offspring. The female may repeat the whole process many times. A week or ten days later the egg hatches and the larva finds a large stock of still-living food immediately available. After coming out of the egg the maggot penetrates the spider’s cuticle and slowly eats it, leaving the vital organs, such as the heart, until last.

The precise and subtle functions of the wasp’s venom are unique in the animal kingdom. If the venom were simply to kill the spider, its tissues would deteriorate before the larva hatched and it would be useless as food. Thus the venom keeps the victim in a state of suspended animation. The second function of the wasp’s venom is that it inhibits the normal decomposition of the tissues while the larva is feeding on these. It is possible because the venom has some antibacterial property. Thus the meat of the spider remains fresh until the wasp larva has consumed it and is ready to pupate.

After it pupates and hatches it emerges as a fully developed adult. If it is female, it digs its way out to find males already waiting for it (perhaps the mother leaves a chemical scent marker). It mates once and never feeds for all its food and energy have come from the spider.

Because of their stinging powers, wasps are generally considered dangerous insects. In fact they are among the most significant of all insects as far as mankind is concerned, owing to their predations on other insects. This is an important factor in checking the world’s already immense insect population.